The primary objectives of this proposed project are to use molecular hybridization techniques to quantitatively measure nucleotide sequence relationships of mammalian RNA tumor viruses including candidate human RNA tumor viruses and to detect virus-related nucleotide sequences in human sarcoma and leukemic cell systems at various levels of nucleic acid expression with nucleic acid probes prepared from mammalian RNA tumor viruses belonging to different virus subgroups. In the human leukemic cell systems, special emphasis will be placed on utilizing virus-specific nucleic acid probes to monitor the clinical course of disease from diagnosis and treatment, through remission, and into relapse. An additional aspect of the proposed project is to employ standard virological procedures to characterize the transformation properties of human sarcoma cell culture systems so that the biological nature of such systems can be correlated with the presence of virus-related nucleotide sequences at the various levels of nucleic acid expression.